1 function 组件

let n = 0;

function App() {
  return (
    <div className="App">{n}
      <button onClick={add}>加一</button>
    </div>
  );
}
function add (){
  n+=1
  render()
}
render()

function render(){
  const rootElement = document.getElementById("root");
  ReactDOM.render(<App />, rootElement)
};

2 class组件

class App extends React.Component {
  constructor(props){
    super(props)
    this.state = {
      n : 0
    }
 }
 add(){
   this.setState({
     n:this.state.n + 1
   })
 }
 render(){
   return(
     <div>{this.state.n}
      <button onClick={this.add.bind(this)}>加一</button>
     </div>
   )
 }
}

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);